The Collaborative International Dictionary of English v.0.48:

Breastplate \Breast"plate`\, n.
   1. A plate of metal covering the breast as defensive armor.
      [1913 Webster]

            Before his old rusty breastplate could be scoured,
            and his cracked headpiece mended.     --Swift.
      [1913 Webster]

   2. A piece against which the workman presses his breast in
      operating a breast drill, or other similar tool.
      [1913 Webster]

   3. A strap that runs across a horse's breast. --Ash.
      [1913 Webster]

   4. (Jewish Antiq.) A part of the vestment of the high priest,
      worn upon the front of the ephod. It was a double piece of
      richly embroidered stuff, a span square, set with twelve
      precious stones, on which were engraved the names of the
      twelve tribes of Israel. See Ephod.
      [1913 Webster] Breastplow

WordNet (r) 3.0 (2006):

breastplate
    n 1: armor plate that protects the chest; the front part of a
         cuirass [syn: breastplate, aegis, egis]